Homodyne receiver



Patented Feb. 11, 1941 UNITED STATES 2,231,704 HOMODYNE RECEIVER Leslie F. Curtis, Great Neck, N. Y.', assignor to Haleltine Corporation, a corporation of Delaware Application March 4, 1939, .Serial No. 259,735

Y 22 Claims.

This invention relates to modulated-carrier signal receivers of the homodyne type and particularly to such receivers in which the 10- cally-generated oscillations are controlled, both as to frequency and to phase, with respect to the carrier wave of the desired received signal.

In a modulated-carrier signal receiver of the homodyne type, locally-generated oscillations of the frequency of the carrier wave of the desired signal are heterodyned with the desired received signal to reproduce the desired modulation signal. If the frequency of the locally-generated oscillations differs slightly from that of the carrier wave of the signal to be reproduced, audible beatfrequency signals result, producing intolerable distortion. Prior art homodynereceivers, therefore, commonly have been provided with an arrangement responsive to the desired received signal for maintaining the desired frequency of the locally generated oscillations. However, such prior art synchronizing arrangements are relatively sensitive to variations in amplitude of received signals, rendering reception unsatisfactory under certain conditions of operation.

Furthermore, even though the frequency of the locally-generated oscillations is exactly maintained at the desired value, considerable distortion in the reproduction of the received signal and a considerable reduction in the volume of the reproduced signal may result, due to an improper phase relationship between the signalcarrier wave and the locally-generated oscillations. It has been determined that there is a critical phase relation between the desired received carrier wave and the locally-generated oscillations at which there is little or no distortion in the reproduced signal. Distortion, due to improper phase relation between these two waves, increases rapidly with departure from this criti- 40 cal or optimum value, while the volume of the reproduced signal simultaneously decreases rapidly. While some prior art homodyne receivers have incorporated in the system, for maintaining the desired frequency of the locallygenerated oscillations, an arrangement the purpose of which is to maintain a constant phase angle between the desired received signal and the locally-generated oscillations, as mentioned above, the operation of such systems of phase control have been materially affected by the amplitude of the received signals. Obviously, with such an arrangement and under such conditions, a phase controlarrangement is almost entirely ineffective.

Furthermore, such systems of the prior art have been ineffective to maintain the critical or optimum phase displacement even under normal conditions of reception, the operation of such receivers thereby being not entirely satisfactory due to distortion caused by improper phase relation between the two high-frequency waves. It is,

therefore, highly desirable to provide a modu lated-carrier signal receiver of the homodyne type in which the frequency of the locally-generated oscillations is maintained at the correct value even though the amplitude of the received signals varies over relatively wide limits and in which there is also provided an additional arrangement for maintaining the phase relationship between the locally-generated oscillations and the desired signal-carrier wave substantially at the optimum value for all values of amplitude of received signals within the synchronizing range of the receiver.

Furthermore, there are present in the detector of most homodyne receivers undesired components of detection of the signal to be reproduced,

which are not properly homodyne detection components and which usually result in a faulty reproduction. However, these undesirable components are reduced materially if a square-law detector is utilized in the receiver. Square-law detectors, however, have a disadvantage in that no automatic amplification control or A. V. C. source is available without the use of additional tubes. It is, therefore, further highly desirable to provide a receiver of the homodyne type comprising a square-law detector in which the above-mentioned critical or optimum phase relationship is maintained and in which an automatic amplification control source is provided without the use of additional tubes. There are also some components of detection, which are undesirable for homodyne reception, which are not eliminated by a detector of the square-law type. These additional undesired components may be eliminated by the use of a balanced detector system.

It is, therefore, an object of the invention to provide an improved modulated-carrier signal receiver ofthe homodyne type in which one or more of the above-mentioned disadvantages of homodyne receivers of the prior art are eliminated.

It is a further object of the invention to provide a modulated-carrier signal receiver of the homodyne type including an arrangement for controlling the phase of locally-generated oscillations with respect to the desired received carrier wave independently of the amplitude of received signals.

In accordance with the invention, a modulated-carrier signal receiver of the homodyne type comprises means for selecting a desired modulated-carrier signal and a local oscillator for generating oscillations of the frequency of the carrier wave of the signal to be reproduced. The receiver also includes means responsive to the frequency-of the carrier wave of a desired received signal, which is preferably relatively insensitive to variations of its amplitude, for maintaining the locally-generated oscillations in synchronism with the first-named carrier wave. There is also provided additional means responsive to the phase difference between the carrier wave of the signal to be reproduced and the locally-generated oscillations for maintaining the above-mentioned phase difference within predetermined limits for all values of amplitude of the received signal within the synchronizing range of the receiver. The receiver additionally includes means for combining the modulatedcarrier signal to be reproduced with the locally generated oscillations to derive the modulationfrequency components of the desired received signal.

In the preferred embodiment of the invention, the phase difference is maintained substantially at the optimum value to minimize distortion and to provide maximum volume of, the reproduced signal. Also, in a preferred embodiment of the invention, the receiver comprises a balanced detector system of the square-law type, thereby to eliminate undesired components of detection. There is preferably provided, with this last-mentioned modification of the invention, means, including the above-mentioned additional means for maintaining the desired phase relationship, for controlling the amplification of the receiver in response to the amplitude of received signals.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing, Fig. 1 is a circuit diagram, partly schematic, of a modulated-carrier signal receiver of the homodyne type incorporating the invention: while Figs. 2a-2f, inclusive, illustrate certain of the operating characteristics of the receiver of Fig. 1 to aid in an understanding of the invention.

Referring now more particularly to Fig. 1 of the drawing, there is shown a modulated-carrier signal receiver of the homodyne type comprising an antenna-ground circuit I0, II coupled to a radio-frequency amplifier I2 through a tunable selector circuit including a variable condenser I3. The radio-frequency output of amplifier" is coupled through transformer I4, I5 to the input circuit of a balanced detector comprising square-law detector tubes l6, I! connected in a balanced circuit. There is also provided a local oscillator for generating oscillations of the frequency of the carrier of the signal to be received.

comprising an oscillator tube l8, a frequencydetermining circuit comprising an inductance l8 and an adjustable tuning condenser 3| coupled to he input grid of tube I8, and a feed-back inductance l9 coupled to an anode grid of tube I8, these grids comprising the scillator electrodes of this tube. The inductance I9 also is coupled to inductance IS, the condenser 3| being connected for unicontrol with condenser I3. An output from oscillator I8 is coupled from inductance I9, by means of inductances 20, 2| coupled thereto, to the input circuits of detector tubes I6 and I! with opposite polarity. The homodyne audiofrequency output of detector tubes I6 and I1 is coupled with .additive' polarity to the input circuit of a low-pass filter 22 through a transformer including primary windings 23, 24 and a secondary winding 25. The low-pass filter 22 may be of any type and is preferably designed to have a high attenuation at 10 kilocycles, the frequency of interfering signals developed from a channel adjacent the channel of the desired received signal. The audio-frequency components of modulation are further amplified in audio-frequency amplifier 25 and are reproduced by loudspeaker 21 in a conventional manner. Unidirectional potentials for tubes I2, I6, I1, and ii are supplied from suitable sources indicated as +3 and +80 in a conventional manner.

The components-of the receiver which have so far been described are well understood in the art, rendering a detailed description of the operation thereof unnecessary. In general, however.

desired signals intercepted by antenna III, II are selected by selector means including variable condenser II, are amplified in radio-frequency amplifier l2, and are applied in the same polarity to each of the detector tubes l6 and I1. Local oscillations generated by oscillator tube I8 of the frequency of the carrier wave of the desired received signal and bearing a predetermined phase relationship to the desired received signal, as will be explained hereinafter in more detail, are coupled with opposite polarity to detector tubes I6 and IT. The audio-frequency modulation signal developed in detector tubes I6 and I1 is passed by low-pass filter 22, amplified in audiofrequency amplifier 26, and reproduced by sound reproducer 21 in a manner well understood in the art.

Coming now to the parts of the system comprising the present invention, transformer I, I5 isprovided with an additional winding 30 for the purpose of applying to a negatively biased grid of tube I8, preferably an electrode additional to the oscillator electrodes of this tube, the desired received signal in order to provide synchronization of the locally-generated oscillations with the carrier wave of the desired received signal through the inherent interelectrode or electronic coupling within tube I8. Transformer I 4, I5, 30 thus comprises means for coupling the carriersignal selecting meanscomprising variable condenser l3 to the additional electrode of tube I 8 to maintain the locally-generated oscillations in synchronism with the carrier wave.' There is also coupled to primary winding I4 an additional winding 32 across which are connected in series a resistor 33 and a condenser 34. Elements 32, 33, and 34, together with primary winding ll, shunted by a condenser 35, comprise a two-terminal dead-end filter forpasslng a wide band of frequencies in a manner fully described in the copending application of Harold A. Wheeler, Serial No. 203,598, filed April 22, 1938, now Patent 2.167.137, dated July 25, 1939. The provision of this broad-band selector renders the provision of a tunable selector at this point of the receiver channel unnecessary.

While the incoming signal, applied to oscillator I8 by means of winding 30, serves to maintain the frequency of the locally-generated oscillations exactly equal to the frequency of the carrier wave of the desired received signals for settings of variable condenser 3| which are close to the correct setting, it is necessary also to provide an additional means for maintaining the phase of the locally-generated oscillations at the optimum relationship with respect to the carrier wave of the received signal. This phase control is provided by the control system 40 including a phasediscriminator circuit including diodes 4| and 42 to which the incoming signal is applied in the same phase through inductances 43 and 44 and to which an output voltage of oscillator I8 is applied to one of the diode rectiflers with one phase and to the other of the diode rectiflers with opposite phase by means of a coupling circuit comprising a condenser 45 and an inductance 46 inductively coupled to inductances 43 and 44 and included in the anode circuit of tube IS. The phase-discriminator circuit is thus jointly responsive to voltages derived from the signal-carrier wave and from the oscillator. Equal load resistors 48 and 49 are provided for diodes 4| and 42, respectively. The differential resultant unidirectional output voltage of diodes 4| and 42 is applied through a filter circuit including series resistors ii, 52 and shunt condensers 53, 54, and 55 to bias the input circuit of a control vacuum tube 56, the purpose of which is to adjust the phase of oscillator I8 to the desired value. A series-connected resistor 51 and blocking condenser 58 are connected between the anode and control grid of tube 56. The anode circuit of tube 56 is connected in shunt to the frequency-determining circuit IO, M and its grid-anode coupling is such that, by proper proportioning of resistor 51 and condenser 58, it simulates a shunt inductance across the circuit l9, ll variable in magni tude in accordance with the control potential applied to its control electrode.

Before considering the operation of the portions of the circuit of Fig. l constituting the present invention, it is desirable flrst to consider the fundamental principles of operation of a homodyne receiver. The components of rectification of such a receiver may be obtained by substituting the sum of the received signal voltages and the voltage of a local oscillator for e in Equation 1 following, which is the equation for the anode current of any homodyne detector tube:

i =Ao+A1e+Aze +Aae Arie" (1) In which i,, :detector output current, A0 to An :coefliclents depending upon the detector tube characteristic, =es+ i+eo:-'the input voltage to the detector.

The voltage es, applied to the detector tube, representing the desired signal, may be expressed by Equation 2 as follows ei=Ei (1'+m cos at) cos wot (2) Where frequency of the carrier wave of the desired 21 received signal and of the local oscillator.

The voltage e1, representing an undesired signal applied to the detector tube, may be expressed by Equation 3 as follows:

er Ea (1+n cos bt) cos wit (3) Where,

E, =peak voltage of the carrier wave of the undesired signal, n=modulation factor of the undesired signal,

pansion contains direct-current; terms, highfrequency terms, and audio-frequency terms. Only those terms representing audio-frequency currents are capable of developing audio-free quency voltages in the detector load circuit: When all other terms are rejected, the expansion of the first three terms of Equation 1 produces Equation 5 representing the audio-frequency plate current:

1 1 cos 2ut r +E n cos bt-i-L: cos 2bt +E,E,1ri cos 4 cos at 1 (5) It will be noted that the first nine terms of the right-hand member of Equation 5 are independent of E0, the oscillator voltage. They may be called the products of ordinary detection, and appear for both desired and undesired signals. The second term of Equation 5 is at double the original desired modulation frequency and represents distortion. The third and fourth terms of Equation 5 are respectively at the fundamental and second harmonic frequencies of the modulation of the undesired signal wave and represent interference. The fifth to twelfth terms, inclusive, of Equation 5 are not ordinarily in the range of frequencies reproduced by the audio-frequency amplifier and loud-speaker. If, however, the desired and undesired signals are on adjacent broadcast channels, the fifth and tenth terms represent the beat response at 10,000 cycles and the sixth, eighth, and eleventh terms or, alternatively, the seventh, ninth, and twelfth terms, depending on whether the interfering signal has a carrier frequency above or below the desired signal, represent additional interferences below 10,000 cycles.

The last term'in Equation 5 depends on the product of the desired signal voltage Es, the modulation factor 1n, and the oscillator voltage E and is, therefore, the desired product of homodyne detection. By making E0 large relative to E; and equal to zero, the last term of Equation becomes greater than the first nine terms,

' thereby materially reducing the percentage of both the distortion and the interference.

The foregoing are the fundamental principles upon which the operation of the simple homodyne receiver is based; that is,'by making the oscillator voltage E0 sufficiently large, the harmonic output is decreased and the ratio of the amplitude of desired signal components to undesired signal components is increased, since the former is produced mostly .by homodyne detection and the latter by detection of the ordinary type.

In the foregoing analysis, the termsbeyond the third term of Equation 1 were not considered. The fourth term contributes no audio-frequency components. The fifth term A46 however, contributes, in addition to the fundamental of the modulation frequency, harmonics up to and including the fourth. It is thus indicated that for a detector to supply components resulting in a minimum of distortion, the cociiicients A4 to An of its characteristic should be negligible. A square-law detector fulfills this condition since its anode current is ideally represented by the first three terms of the right-hand member of the system; that is, Equation 5 Equation 1. For this reason detector tubes l5 and I! are preferably of the square-law type.

An improved detector circuit is provided by the use of the balanced detector system of Fig. 1. In the ideal case, that is, with a perfectly balanced square-law detector system, all except the last four terms of Equation 5 are eliminated by cancellation in the push-pull output circuit of becomes:

Inasmuch as an undesired signal on an adja cent channel produces in the detector output a heterodyne frequency equal to the difference fre quency between the local oscillator and the undesired signal, represented by the first term of Equation 6, a low-pass filter 22, designed to have a high attenuation of kilocycles, is preferably provided immediately following the detector. Either the second or the third term of Equation 6 may contain interfering modulation-frequency terms within the range of audio-frequency reproduction, if the interfering signal is on a channel adjacent to the desired signal, but, in this case, the interfering signal component is within the band of desired signals and may not be eliminated by any means which does not also reduce the response at the desired signal frequencies. As a result of the discrimination against undesired signals provided by balanced homodyne detection and by the low-pass filter 22, the selectivity requirements of the radio-frequency channel of the receiver of Fig. 1 are greatly reduced so that the single tunable selector circuit comprising variable condenser I3 is sufficient.

When (am-ml) /2rr is greater than the frequency separation between adjacent channels, the first three terms of Equation 6 represent components above audibility and the equation reduces to:

i=2EoEsm cos cos a't tional to the amplitudes of the signalcarrier and local oscillator voltages and to the cosine of the phase angle between them. when the phase an gle is zero, the output is a maximum and when it is w/Z, the output is zero.

Reference is now made to Figs. 2a2/, inclusive, the curves of which are all plotted against values of condenser 3| as abscissae, for a complete explanation of the operation of the circuit of Fig. 1. In the absence of a synchronizing voltage on the tube of oscillator l8, the variation of the oscillator frequency as a function of capacitance C of tuning condenser 3| is given by the dashed line in Fig, 21. For the value Co of the condenser C, the frequency of the oscillator is assumed to be is. If, under these conditions, a synchronizing voltage of frequency in is applied to the grid of tube I8 through inductance 30, the oscillator frequency remains unchanged. Furthermore, as tuning condenser 3| is adjusted within the limits CO+AC and CAC, the oscillator frequency remains at the desired frequency In. This operation is indicated by the solid horizontal line in Fig. 2a. Beyond the values CoiAC, however, synchronization is not maintained and the full-line curve of Fig. 2a becomes asymptotic t0 the dashed line. The greater the synchronizing voltage applied through inductance 30, the wider the region 243C over which synchronization is maintained although, as compared to most synchronizing arrangements of the prior art, the synchronizing arrangement of the invention is relatively insensitive to changes in amplitude of the desired received carrier.

The diagrams of Fig. 2b illustrate the phase relations which exist between several of the circuit voltages and the anode current as a function of the tuning capacitance C. The voltages ez, ea, and er, respectively, represent the radiofrequency carrier voltage applied through inductances 20 and 2| to the input electrodes of detector tubes l6 and H, the carrier voltage applied through inductances 43 and 44 to the phasecontrol diodes 4i and 42, and the carrier voltage applied from winding 30 to oscillator I 8. These voltages all have the same phase which, for convenience, has been selected as the reference phase, and is and e1 represent, respectively, the anode current of oscillator l8 and the voltage across the frequency-determining circuit l9, 3| as determined by tests of a typical circuit. As shown by Fig. 2b when 0:00, the current i8 is in phase with the synchronizing voltage e4 while e1 is opposite in phase to e4.- As C is decreased by AC, ia lags e; by 1r/2. The central portions of the phase curves are approximately linear so that, for these regions, the phase angles are substantially proportional to variations in the value of condenser 3i.

As will be explained more in detail hereinafter, it is necessary for the oscillator voltage 61), applied to. the phase-discriminator circuit including diodes 4| and 42, to be in quadrature with the signal-frequency voltage (23, also applied thereto, when the value of capacitance of tuning condenser 3| is C=Co. Since the anode current i. is then in phase with said signal voltage 63, the desired phase relation for E11 may be obtained by allowing the current is. to fiow through a reactive load circuit. This is provided by condenser 45 and inductance 46 adjusted to resonate below the band of frequencies to be received by the receiver. The resulting phase relation for the voltage e is also shown in Fig. 2b.

As explained above, the rectified output of a balanced homodyne detector is proportional to the cosine of the phase angle at the detector between the locally-generated oscillations and the sponding to values of tuning condenser ll of C+AC and Co--AC, respectively, which would exist if no-control were present. Outside the region between CoiAC, the oscillator is unsynchronized and modulation-frequency signals are masked by'beat-frequency signals developed between the signal-carrier wave and the local oscillations.

Fig. 2d illustrates the variation of distortion as a function of variation of capacitance C, primarily due to lack of perfect balance. that the maximum distortion is obtained forthe condition giving minimum volume and vice versa, as shown by Fig. 20. Tests indicate that phase modulation in the oscillator also contributes materially to the production of the distortion indicated by the curve of Fig. 2d.

It is evident from Figs. 2c and 2d that the desired setting of condenser 3| must be such that the deviation of its capacitance from the value Co is small and preferably zero. Since, in manually varying tuning condenser 3i, it can only be expected that the oscillator will be brought to some point within the range of synchronization and not necessarily to the exact point required for optimum signal reproduction, an automatic means is required for decreasing the eifective departure of the value of condenser 3i from its desired value. This is accomplished by means of unit 40 of Fig. l. The resultant unidirectional voltage output developed by the phase-responsive 1 circuit including diodes ll and 42 is shown in Fig. 2e. By a method similar to that used for obtaining the audio-frequency components, it may be shown that the direct current in diode ll is given by Equation 8 as follows:

1 2 i,=A;[ g l-TE;E, cos il (8) Where Az=the coefiicient in current, Ez=peak voltage of the carrier wave of the signal, E =peak anode voltage of the oscillator,

T=vo1tage transformation ratio of transformer 46, 43, M, and =phase angle between In a similar way the direct current is given by Equation 9 as follows:

the expression for diode the above voltages.

in diode 42 Since these currents develop unidirectional voltages in resistors 48 and H which are in opposition, the net unidirectional voltage output of the two diodes is:

Ec=2RA2TE3Ep 005 il (10) where R. is the resistance of each of resistors 48 and I9.

If q is made 11/2 when the value of C=C, then the voltage Ec is zero but assumes increasing positive or negative values for departures of C from C0. Since a is arranged to lag i. by 1r/2, and is the angle between the oscillator and signal voltages at the signal detector, Equation 10 may be rewritten as Equation 11 as follows:

E0: :2RAzTEaE sin 4: (11) It will be noted The tube'lifi, biased by unidirectional voltage Ea, therefore, supplies an increasingly larger eflective susceptance for phase correction as C departs fromCo. The action of this susceptance, supplied by the control tube 56, is very similar to that of the operation upon the frequency-determining circuit of the local oscillator of the corresponding tube of an automatic frequency control system of a conventional superheterodyne receiver.

By way of illustration, if E0 is made negative, due to an arbitrary setting of C=Co-AC1, the negative susceptance which the control tube 58 supplies across the tuned circuit l9, 3| is less than for C=C. This reduces the resonant frequency of the circuit 19, 3| plus the control tube 56. A similar change in tuning might have been obtained by manually increasing the value of C, which, as shown by Fig. 2b, brings the voltages more nearly to the desired phase. The action continues until the effective remaining departure-A'Cr is just sumcient to produce the voltage E1 required to shift the equivalent effective capacitance by an amount ACi-A'Ci.

Fig, 2 shows this relationship. The dashed diagonal line of Fig. 2f provides a means for transferring the values of AC to the scale of abscissae of Figs. 2a -2e, the value AC1 on the scale of abscissae being indicated in Fig. 2!. The vertical line through AC1 represents the conditions in Figs. 2'b-2f, inclusive, for the illustrative manual setti 8 alone. The vertical line drawn through -Eci represents the conditions in Figs. 2b-2f, inclusive, after the automatic control acts.

If the original manual setting had been for a value of C=CO+AC1, the operation would have been similar except that the developed voltage Ec would have been positiveand the shift in tuning would have been in the opposite direction. In other words, the phase of oscillator voltage input to detectors l5, l1 remains nearly constant (0 or r radians) with respect to the signal-inputvoltage 0: thereto, as the value of condenser 3i is adjusted between the limits CoiAC, which is the range of synchronization. Specifically, for a setting of condenser 3| corresponding to a value of Co-ACI, it is seen that the actual variation of the eflective value of condenser 3! from its desired only a small amount while the volume is near its maximum value.

It is thus seen that the phase-controlling arrangement of the invention is effective to reduce the deviation of the uncontrolled phase difference between the desired received signal and the locally-generated oscillations from its optimum value by at least 2:1 for values of said uncontrolled phase diiference displaced with respect to the optimum value of the phase difference for distortionless reception, as shown by the phase difterences corresponding to AC1 and -AC1 on the curves of Figs. 2b-2j, inclusive, and that the device is thus effective to maintain the controlled phase difference substantially within the range of 11/4 with respect to the optimum value for distortionless reception. In the preferred embodiment of the invention, the above-mentioned reduction is much greater than 2:1, being at least of the order of 10:1. It is seen from curve 20, therefore, that the volume output of the receiver remains substantially constant and, from curve 241, that the distortion remains near the minimum value over the operating range of the system. Outside the range of synchronization, the phasecontrol circuits exert no controlling influence on the circuit of the oscillator. In the region outside the range of synchronization, beat-frequency signals are produced between the oscillator and the desired signal voltages and unsatisfactory operation results.

It will be understood that various silencing arrangements known to the prior art can be utilized to silence the receiver while tuning and that the automatic phase-control arrangement of the invention can be made to be inoperative while tuning the receiver.

Inasmuch as no satisfactory source of automatic volume control is available in the balanced detector of Fig. 1, the voltage developed across diode load resistor 49 of the phase-discriminator unit is utilized as a control bias and is applied to the input electrode of tube l2 and any other similar stages that may be included in the receiver, thereby to maintain the signal input to detectors I6 and I! within a relatively narrow range of amplitude for a wide range of received signals, in a manner well understood in the art.

While applicant does not wish to be limited to any particular circuit values for the embodiment of the invention described, there follows a set of representative values which may be utilized in the circuit of Fig. 1:

Tube l8 Type 6A8 Tubes 4| and 42 Type 6H6 Tube 56 Type 6J7 Tubes l6 and I1 'Iype 6J7 Inductance 32 1.6 millihenries Inductance I 4 1.6 millihenries- Inductance |5 20 microhenries Coeflicient of coupling between inductances I4 and 32 0,8

Coeflicient of coupling between inductances I4 and i5 0.12 Capacitance 30 micro-microfarads Inductance 46 10 millihenries Inductance 43 .5 millihenries Inductance 44 -5 millihenries Coefllcient of coupling between inductance 46 and inductances 43 and 44 v0.8 Resistor 48 0.5 megohm Resistor 49 0.5 megohm Resistor 52 1 megohm Resistor 51 .50,000 ohms Capacitance 58 .0.001 microfarad Capacitance 100 micro-microfarads Inductance l9 200 microhenries Inductance l9 -20O microhenries Inductance 20 35 microhenries Inductance 2| .35 microhenries Coefficient of coupling between inductance l9 and inductances 20 and While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is; therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A modulated-carrier signal receiver of the homodyne type comprising, means for selecting a desired modulated-carrier signal, a local oscillator for generating oscillations of the frequency of the carrier wave of the signal to be reproduced, means responsive to the frequency of the carrier wave of a desired received signal for maintaining said locally-generated oscillations in synchronism with said first-named carrier wave, additional means responsive to the phase diflerence between the carrier wave of the said signal to be reproduced and said locally-generated oscillations for maintaining said phase difference within predetermined limits, and means for combining the modulated-carrier signal to be reproduced and said locally-generated oscillations to derive the modulation-frequency components 01' said desired received signal.

2. A modulated-carrier signal receiver of the homodyne type comprising, means for selecting a desired modulated-carrier signal, a local oscillator for generating oscillations of the frequency of the carrier wave of the signal to be reproduced, means responsive to the frequency of the carrier wave of a desired received signaland relatively insensitive to variations of its amplitude for maintaining said locally-generated oscillations in synchronism with said first-named carrier wave, additional means responsive to the phase diiierence between the carrier wave of the said signal to be reproduced and said locally-generat ed oscillations for maintaining said phase difference within predetermined limits, and means for combining the modulated-carrier signal to be reproduced and said locally-generated oscillations to derive the modulation-frequency components of said desired received signal.

3. A modulated-carrier signal receiver of the homodyne type comprising, means for selecting a desired modulated-carrier signal, a local oscillator for generating oscillations of the frequency of the carrier wave of the signal to be reproduced, means responsive to the frequency of the said carrier wave and substantially independent of its amplitude. for maintaining the said locallygenerated oscillations in synchronism with said carrier wave, additional means responsive to the phase diiference between the said carrier wave and said locally-generated oscillations for maintaining said phase difierence substantially at the optimum value for distortionless reproduction,

and means for combining the modulated-carrier signal to be reproduced and said locally-generated oscillations to derive the modulationfrequency components of said desired received signal.

4. A modulated-carrier signal receiver of the homodyne type comprising, means for selecting a desired modulated-carrier signal, a local oscillator for generating oscillations of the frequency of the carrier wave of the signal to be reproduced, means responsive to the frequency of the carrier wave of a desired received signal for maintaining said locally-generated oscillations in synchronism with said firstmamed carrier wave, additional means responsive to the phase diflerence between the carrier wave of the signal to be reproduced and said locally-generated osclllations to derive the modulation-frequency com ponents of said desired received signal.

5. A modulated-carrier signal receiver of the homodyne type comprising, means for selecting a desired modulated-carrier signal, a local oscillator for generating oscillations of the frequency of the carrier wave of the signal to be reproduced, means responsive to the frequency of the carrier wave of a desired received signal for maintaining said locally-generated oscillations in synchronism with said first-named carrier wave, additional means responsive to the phase difference between the carrier wave of the signal to be reproduced and said locally-generated oscillations for reducing the deviation of the uncontrolled phase difference from its optimum value by a ratio of at least two to one for values of said uncontrolled phase difference substantially displaced with respect to the optimum value of said phase difference for distortionless reception, and means for combining the modulated-carrier signal to be reproduced and said locally-generated oscillations to derive the modulationfrequency components of said desired received signal.

6. A modulated-carrier signal receiver of the homodyne type comprising, means for selecting a desired modulated-carrier signal, a local oscillator for generating oscillations of the frequency of the carrier wave of the signal to be reproduced, means responsive to the frequency of the carrier wave of a desired received signal for maintaining said locally-generated oscillations in synchronism with said first-named carrier wave, additional means responsive to the phase difference between the carrier wave of the signal to be reproduced and said locally-generated oscillations for reducing the deviation of the uncontrolled phase difference from its optimum value by a ratio of at least ten to one for values of said uncontrolled phase difference substantially displaced with respect to the optimum value of said phase difference for distortionless reception, and means for combining the modulated-carrier signal to be reproduced and said locally-generated oscillations to derive the modulation-frequency components of said desired received signal.

7. A modulated-carrier signal receiver of the homodyne type comprising, means for selecting a desired modulated-carrier signal, a local oscillator including a vacuum tube for generating oscillations of the frequency of the carrier wave of the signal to be reproduced, means for electronically coupling said selecting means to said vacuum tube for maintaining said locally-generated oscillations in synchronism with said firstnamed carrier wave, additional means responsive to the phase difference between the carrier wave of the said signal to be reproduced and said locally-generated oscillations for maintaining said phase difference within predetermined limits, and means for combining the modulatedcarrier signal to be reproduced and said locallygenerated oscillations to derive the modulationfrequency components of said desired received signal.

8. A modulated-carrier signal receiver of the homodyne type comprising, means for selecting a desired modulated-carrier signal, a local oscillator including a vacuum tube having oscillator electrodes and an additional electrode, said oscillator being provided to generate oscillations of the frequency of the carrier wave of the signals to be reproduced, means forcoupling said selecting means to said additional electrode to maintain said locally-generated oscillations in synchronism with said carrier wave, additional means responsive to the phase difference between the carrier wave of the said signal to be reproduced and said locally-generated oscillations for maintaining said phase difference within predetermined limits, and meansfor combining the modulated-carrier signal to be reproduced and said locally-generated oscillations to derive the modulation-frequency components of said desired received signal.

9. A modulated-carrier signal receiver of the homodyne type comprising, means for selecting a desired modulated-carrier signal, a local oscillator for generating oscillations of the frequency of the carrier wave of the signal to be reproduced, means responsive to the frequency of the carrier wave of a desired received signal for maintaining said locally-generated oscillations in synchronism with said first-named carrier wave, means for deriving a first voltage from said oscillator and a second voltage from said desired received signal, a phase-discriminator means responsive jointly to said first and second voltages for maintaining the phase difference between the carrier wave of the signal to be reproduced and said locally-generated oscillations within predetermined limits, and means for combining the modulated-carrier signal to be reproduced and said locally-generated oscillations to derive the modulation-frequency components of said desired received signal.

10. A modulated-carrier signal receiver of the homodyne type comprising, means for selecting a desired modulated-carrier signal, a local oscillator for generating oscillations of the frequency of the carrier wave of the signal to be reproduced, means responsive to the frequency of the carrier wave of a desired received signal for maintaining said locally-generated oscillations in synchronism with said first-named carrier wave, two rectifiers, means for applying an output voltage from said oscillator and a voltage derived from said desired received signal to said rectifiers, one of said voltages being applied in the same phase to each of said rectifiers and the other of said voltages being applied to one of said rectifiers with one phase and to the other of said rectiflers with opposite phase, means responsive to the differential output of said rectifiers for maintaining the phase difference between the carrier wave of the signal to be reproduced and said locally-generated oscillations within predetermined limits, and means for combining the modulated-carrier signal to be reproduced and said locally-generated oscillations to derive the modulation-frequency components of said desired received signal. I

11. A modulated-carrier signal receiver of the homodyne type comprising, means for selecting a desired modulated-carrier signal, a local oscillator for generating oscillations of the frequency of the carrier wave of the signal to be reproduced, means responsive to the frequency of the carrier wave of a desired received signal for maintaining said locally-generated oscillations in synchronism with said first-named carrier wave, means for deriving a first voltage from said oscillator and a second voltage from said desired received signal, phase-discriminator means responsive to said first and second voltages for developing a bias voltage varying in accordance with the difference in phase between said first and said second voltages, means for utilizing said bias to maintain the phase difference between the carrier wave of the signal to be reproduced and said locally-generated oscillations within predetermined limits, and means for combining the modulated-carrier signal to be reproduced and said locally-generated oscillations to derive the modulation-frequency components of said desired received signal.

12. A modulatedcarrier signal receiver of the homodyne type comprising, means for selecting a desired modulated-carrier signal, a local oscillator for generating oscillations of the frequency of the carrier wave of the signal to be reproduced, means responsive to the frequency of the carrier wave of a desired received signal for maintaining said locally-generated oscillations in synchronism with said first-named carrier wave,

two rectifiers, means for applying an output voltage from said oscillator and a voltage derived from said desired received signal to said rectifiers, said voltages being relatively displaced 90 degrees in phase under normal operating conditions of said system and one of said voltages being applied in the same phase to each of said rectifiers and the other of said voltages 'being applied to one of said rectifiers with one phase and to the other of said rectifiers with opposite phase, means responsive to the differential output of said rec-1ifiers for maintaining the phase difference between the carrier wave of the signal to be reproduced and said locally-generated oscillations within predetermined limits, and means for combining the modulated-carrier signal to be reproduced and said locally-generated oscillations to derive the modulation-frequency components of said desired received signal.

13. A modulated-carrier signal receiver of the homodyne type comprising, means for selecting a desired modulated-carrier signal, a local oscillator including a frequency-determining circuit for generating oscillations of the frequency of the carrier wave of the signal to be reproduced, means responsive to the frequency of the carrier ,wave of a desired received signal for maintaining said locally-generated oscillations in synchronism with said first-named carrier wave, two diode rectifiers, means for applying an output voltage from said oscillator and a voltage derived from said desired received signal to said rectifiers,one of said voltages being applied in the same phase to each of said rectifiers and the other of said voltages being applied to one of said rectifiers with one phase and to the other of said rectifiers with opposite phase, means responsive to the differential output of said rectifiers effectively varying the reactance of said frequency-determining circuit to maintain the phase difference between the carrier wave of the said signal to be reproduced and said locallygenerated oscillations within predetermined limits, and means for combining the modulatedcarrier signal to be reproduced and said locallygenerated oscillations to derive the modulationfrequency components of said desired received signal.

14. A modulated-carrier signal receiver of the derived from said desired received signal to said rectifiers, one of said voltages being applied in the same phase to each of said rectifiers and the other of said voltages being applied to one 01' said rectifiers with one phase and to the other of said rectifiers with opposite phase, electronic reactance means controlled by the differential output of said rectifiers and coupled to said frequency-determining circuit to maintain the phase diii'erence between said carrier wave of said signal .to be reproduced and said locally-gcnerated oscillations within predetermined limits, and means for combining the modulated-carrier signal to be reproduced and said locally-generated oscillations to derive the modulation-frequency components of said desired received signal.

15. A modulated-carrier signal receiver of the homodyne type comprising, means for selecting a desired modulated-carrier signal, a local oscillator for generating oscillations of the frequency of the carrier wave of the signal to be reproduced,

applying an output voltage from said oscillator and a voltage derived from said desired received signal to said rectifiers to develop a bias voltage varying in accordance with the phase difference of said last-mentioned voltages, means for utilizing said bias voltage to maintain the phase difference between the carrier wave of the signal to be reproduced and said locally-generated oscillations within predetermined limits, means comprising said load circuit for maintaining the volume of the reproduced signal within relatively narrow limits for a wide range of received signal amplitudes, and means for combining the modulated-carrier signal to be reproduced and said locally-generated oscillations to derive the modulation-frequency components of said desired received signal.

16. A modulated-carrier signal receiver of the homodyne type comprising, means for selecting a desired modulated-carrier signal, a local oscillator for generating oscillations of the frequency of the carrier wave of the signal to be reproduced, means responsive to the frequency of the carrier wave of a desired received signal for maintaining said locally-generated oscillations in synchronism with said first-named carrier, wave, additional means responsive to the phase difference between the carrier wave of the said signal to be reproduced and said locally-generated oscillations for maintaining said phase difference within predetermined limits, and means including a square-law detector for combining the modulated-carrier signal to be reproduced and said locally-generated oscillations to derive the modulation-frequency components of the desired received carrier signal.

17. A modulated-carrier signal receiver of the homodyne type comprising, means for selecting a desired modulated-carrier signal, a local oscillator for generatingoscillations of the frequency of the carrier wave of the signal to be reproduced, means responsive to the frequency of the carrier wave of a desired received signal for maintaining said locally-generated oscillations in synchronism with said first-named carrier wave, additional means responsive to the phase difi'erence between the carrier wave of said signal to be reproduced and said locally-generated oscillations for maintaining said phase diflference within predetermined limits, two vacuum-tube detectors, means for applying a voltage derived from said signal to be reproduced and a voltage derived from the output of said oscillator to each of said detectors, one of said voltages being applied to both of said detectors in the same phase and the other of said voltages being applied to one of said detectors in one phase and to the other of said detectors in opposite phase, thereby to derive the modulation-frequency components of said desired received signal.

18. A modulated-carrier signal receiver of the homodyne type comprising, means for selecting a desired modulated-carrier signal, a local osci1- later for generating oscillations of the frequency of the carrier wave of the signal to be reproduced, means responsive to the frequency of the carrier wave of a desired received signal for maintaining said locally-generated oscillations in synchronism with said first-named carrier wave, additional means responsive to the phase difference between the carrier wave of said signal to be reproduced and said locally-generated oscillations for maintaining said phase difference within predetermined limits, two vacuum-tube detectors, means for deriving a voltage from said signal to be reproduced and a voltage from the output of said oscillator, said voltages being substantially of the same phase and one of said voltages being applied to both of said detectors in the same phase and the other of said voltages being applied to one of said detectors in one phase and to the other of said detectors in opposite phase, thereby to derive the modulation frequencies of the said desired received signal.

19. A modulated-carrier signal receiver of the homodyne type comprising, means for selecting a desired modulated carrier signal, a local oscillater for generating oscillations of the frequency of the carrier wave of the signal to be reproduced, means responsive to the frequency of the carrier wave of a desired received signal for maintaining said locally-generated oscillations in synchronlsm with said first-named carrier wave, additional means responsive to the phase difference between the carrier wave of the said signal to be reproduced and said locally-generated oscillations for maintaining said phase difference within predetermined limits, two square-law vacuumtube detectors, and means for applying a voltage derived from the desired received signal and a voltage derived from the output of said oscillator to each of said detectors, one of said voltages being applied to both of said detectors in the same phase and the other of said voltages being applied to one of said detectors in one phase and to the other of said detectors in opposite phase, thereby to derive the modulation frequencies of said desired received signal.

20. A modulated-carrier signal receiver of the homodyne type comprising, means for selecting a desired modulated-carrier signal, a local oscillator for generating oscillations of the frequency of the carrier wave of the signals to be reproduced, means responsive to the frequency of the carrier wave of a desired received signal for maintaining said locally-generated oscillations in synchronism with said first-named carrier wave, additional means responsive to the phase diiference between the carrier wave of the said signal to be reproduced and said locally-generated oscillations for maintaining said phase difference within predetermined limits. a signal detector, means for applying a voltage derived from a desired received signal and a voltage derived from the output of said oscillator to said detector to derive the modulation frequencies of said desired received signal, and a low-pass filter coupled to the output circuit of said detector to,

attenuate undesired signals appearing in channels adjacent that of the desired signal.

21. A modulated-carrier signal receiver of the homodyne type comprising, means for selecting a desired modulated-carrier signal, a signal-transmitting channel including one or more amplifier stages, a local oscillator including a tunable frequency-determining circuit for generating oscillations of the frequency of the carrier wave of the signal to be reproduced, a tunable signal selector included in said channel preceding the first amplifier of said receiver, unicontrol tuning means for said tunable frequency-determining circuit and said tunable selector, means responsive to the frequency of the carrier wave of the desired received signal for maintaining said locally-generated oscillations in synchronism with said first-named carrier wave, additional means responsive to the phase difference between the carrier wave of said signal to be reproduced and said locally-generated oscillations for maintaining said phase difference within predetermined limits, a detector in said signal-translating channel, and a low-pass filter in said signaltranslating channel at a point succeeding said detector to eliminate undesired signals appearing in channels adjacent to the channel of said desired received signal.

22. A modulated-carrier signal receiver of the homodyne type comprising, means for selecting a desired modulated-carrier signal, a local oscillator for generating oscillations of the frequency of the carrier wave of the signal to be reproduced, means responsive to the frequency of the carrier wave of a desired received signal for maintaining said locally-generated oscillations in synchronism with said first-named carrier wave, two rectiflers, means for applying an output voltage from said oscillator and a voltage derived from said desired received signal to said rectiflers, one of said voltages being applied in the same phase to both of said rectiflers and the other of said voltages being applied to one of said rectifiers with one phase and to the other of said rectifiers with opposite phase, means responsive to the differential output of said rectiiiers for maintaining the phase difference between the carrier wave of the signal to be reproduced and said locally-generated oscillations within predetermined limits, two vacuum-tube detectors, means for applying a voltage derived from said signal to be reproduced and a voltage derived from the output of said oscillator to each of said detectors, said last-named voltage derived from said oscillator and said voltage derived from said signal to be reproduced being 90 degrees displaced in phase, and one of said voltages applied to said detectors being applied to both of said detectors in the same phase and the other of said voltages applied to said detectors being applied to one of said detectors in one phase and to the other of said detectors in opposite phase, thereby to derive modulationfrequency components of said desired received 

